In this way, Ref. [1]} by unitarizing the next-to-leading order (NLO) \(\pi \pi \)  scattering amplitude in partial-wave amplitudes (PWAs) obtained the pole position of the \(\sigma \)  resonance at \(\sqrt{s_\sigma }=458\pm 14-i\,(261\pm 17)\)  MeV. This pole position is compatible at the level of one sigma with \(\sqrt{s_\sigma }=457^{+14}_{-13}-i\,(297^{+11}_{-7})\)  MeV, the value obtained by solving Roy-like equations [2]} (the so-called GKPY equations). It is also clear from the fact that \(|s_\sigma /(4\pi f_\pi )^2|\approx 0.22\ll 1\)  that the \(\sigma \)  meson should be actually considered as a low-energy degree of freedom in the EFT and should be accounted for. This point is certainly achieved by employing unitarization techniques, so that this resonance emerges dynamically as a result of the pion interactions.
As explained in Ref. [3]}, there is a parametrical enhancement affecting the leading-order (LO) isoscalar-scalar \(\pi \pi \)  PWA which causes the emergence of the \(\sigma \)  resonance.
